How do I remove the line after merged the 2 objects together?
I would go into edit mode and look for duplicate vertices and merge them. Then check your edge flow and see if those that are there need adjustment, and maybe add/remove some loops in that area. It looks like it's really pinched in there, which is streatching the existing polygons, which in turn stretches your shading. AS a low poly model it won't do much, but as you subdivide, Blender will make calculations based on the existing geomotry that is there, so that's where I would start. Good luck :)